I've noticed quite a bit more graphic details in their recent product pages and sign up forms. Turns out their new designers do indeed design in Photoshop:
http://37signals.com/svn/posts/1821-work-in-photoshop-dont-s...
When you're adding a lot of visuals, you'll end up using them in the final product, which is where designing that stuff and laying it out in Photoshop becomes more practical. It really depends on the style of the UI and how heavily it relies on custom images.